Updates on mechanisms of maintenance of species diversity

A quantitative approach to species coexistence based on the invasibility criterion has led to an appreciation of coexistence mechanisms in terms of stabilizing and equalizing components, but major challenges are the need to consider general multispecies settings, interactions beyond competition, and multiple scales of space and time. Moreover, two essential concepts, species-level average fitness and scaling factors, have not had clear definitions. A general approach to defining average fitnesses and scaling factors is given, along with the origin of stabilizing mechanisms as deviations from a reference model where no coexistence is possible. Illustrations are general Lotka–Volterra models, models accounting specifically for resource use and natural enemies, and models with temporal fluctuations. Community averages of stabilizing mechanisms reveal overall opportunities for coexistence, and define mechanisms more precisely through their formulae. Average fitnesses adjusted for the presence of coexistence mechanisms provide a better definition of equalizing mechanisms. While these ideas apply to the components of invasion rates, permanence theory and stochastic persistence theory show how invasion rates can be used to demonstrate species coexistence in complex settings. Although species coexistence has often focused on competition, detailed models of the roles of natural enemies provide a new perspective on the opportunities for coexistence in nature. The concept of apparent competition recognizes the essential symmetry between density-dependence from resource depletion and from supporting natural enemies. Natural enemy partitioning is the natural analogue of resource partitioning and has an equivalent role in promoting coexistence. Rather than reinforcing each other, however, the strength of coexistence is often intermediate between that implied by resource partitioning alone and that implied by natural enemy partitioning alone, as elucidated by recent Lotka–Volterra theory. Synthesis. Although there are alternative approaches for understanding coexistence in multispecies settings, ideas based on stabilizing and equalizing mechanisms continue to provide new insights. Multiple species and multiple trophic levels are naturally challenging, but the new theories of permanence and stochastic persistence support the critical role of invasion rates in species coexistence, and thus support the understanding to be derived by partitioning invasion rates into average fitness differences and stabilizing components.